1. Field of the Invention
The present invention relates generally to a glass composition (a clad glass composition) that forms a clad of a mother glass rod for gradient-index rod lens having a core/clad structure, and a mother glass rod for a gradient-index rod lens formed using the same. The present invention also relates to a gradient-index rod lens formed of the mother glass rod and a method of manufacturing the same as well as a rod lens array and an image processor that include the rod lens.
2. Related Background Art
A gradient-index rod lens (hereinafter also referred to simply as a “rod lens”) is a rod-shaped optical element having a refractive index distribution from its center to its periphery in the section thereof. The rod lens can exhibit lens functions such as, for instance, an imaging function even when both the end faces thereof are flat, and has many features including one that facilitates the production of lenses with minute diameters, for instance. Using such features, a rod lens array in which the rod lenses described above are arrayed is employed for a wide range of uses as an imaging lens in an optical system of an image processor (for example, an image-forming apparatus such as an LED array printer, a liquid crystal shutter printer, etc, or an image scanner such as a facsimile, a scanner, etc.).
The gradient-index rod lens usually is formed by providing a mother glass rod for a gradient-index rod lens (hereinafter also referred to simply as a “mother glass rod”) with a refractive index distribution. The mother glass rod is manufactured by a direct drawing method (a continuous drawing method) that has been used widely due to its excellent productivity. In the direct drawing method, as shown in FIG. 10, molten glass 101 that has been subjected to a degassing process (a fining process) as required is put into a crucible 103 that is kept warm with a heater 102. The molten glass 101 flows out of the lower end of a nozzle 104 located in a lower end portion of the crucible 103 while being cooled gradually inside the nozzle 104. Thereby a mother glass rod (a fiber) 105 with a diameter of about 0.1 mm to 4 mm can be formed continuously through hot drawing. In this method, the drawing speed (a formation speed) at which the mother glass rod 105 is drawn can be several tens of meters per minute while the crucible 103 can be charged with the molten glass continuously. Thus the productivity in manufacturing the mother glass rod can be increased.
The direct drawing method, however, has a disadvantage that the molten glass 101 tends to devitrify when being cooled gradually inside the nozzle 104. Particularly, molten glass that is free from lead (whose typical composition is PbO) tends to devitrify when being drawn by the direct drawing method using a crucible.
In order to prevent such devitrification from occurring during the drawing, for instance, JP8 (1996)-13691A discloses a method in which additives such as BPO4, Al2(PO3)3, etc. are added to molten glass.
Furthermore, JP5 (1993)-72337A discloses a method of forming a mother glass rod 118 having a structure (a core/clad structure) composed of a core 116 and a clad 117 covering the core 116 by the direct drawing method (a concentric crucible drawing method) using a concentric crucible 115 including an inner crucible 113 and an outer crucible 114 (see FIG. 11). The inner crucible 113 and the outer crucible 114 are charged with a core glass composition 111 that forms the core 116 and a clad glass composition 112 that forms a clad 117, respectively, with the core glass composition 111 and the clad glass composition 112 being in a molten state. The respective glass compositions are allowed to flow out from a nozzle 119 located in the lower end portion of the concentric crucible 115 to form the mother glass rod 118. The mother glass rod 118 thus formed can be drawn out continuously with rollers 120. In this case, when a glass composition that tends not to cause devitrification is employed as the clad glass composition 112, the core glass composition 111 can be prevented from coming into contact with the nozzle 119 even in the temperature region where devitrification tends to occur in the case of using the direct drawing method. Accordingly, the core glass composition 111 can be prevented from devitrifying and thus the whole mother glass rod 108 can be prevented from devitrifying during the drawing.
JP2004-151682A discloses an example of the clad glass composition that tends not to cause devitrification, specifically:
a clad glass composition of a mother glass rod for a gradient-index rod lens that is substantially free from PbO (unless containing both TiO2 and La2O3),
wherein the clad glass composition is substantially free from PbO and substantially includes the following components, indicated by mol %:
45 to 65% SiO2;
3 to 30% Na2O;
0 to 10% K2O;
0 to 15% MgO;
0 to 20% BaO;
0 to 15% B2O3,
0 to 10% ZnO;
0 to 10% TiO2;
0 to 7% Y2O3;
0 to 7% ZrO2;
0 to 7% Nb2O5;
0 to 7% In2O3;
0 to 7% La2O3; and
0 to 10% Ta2O5,
where the total of Na2O+K2O is in the range of 3 to 35 mol %,
the total of MgO+BaO is in the range of 0 to 25 mol %, and
the total of B2O3+ZnO+TiO2+Y2O3+ZrO2+Nb2O5+In2O3+La2O3+Ta2O5 is in the range of 0 to 20 mol %.